Inhalation anaesthetics are drugs that are breathed into the lungs and from there absorbed into the blood. Inhalation anaesthetics are either gases (such as nitrous oxide, helium, etc) or vapours. Vapours are usually produced by volatile liquids, which primarily exist as a liquid at room temperature. Inhalation anaesthetic agents that are vapours include halothane, isoflurane, enflurane and sivoflurane.
To facilitate administration of anaesthetic vapours, it is usually necessary to encourage the liquid to form a vapour or gas. This vapour is administered in a controlled fashion to an anaesthetic breathing circuit, and therefore to the patient. The devices which hold the liquid anaesthetic and produce a vapour are called anaesthetic vaporisers. Anaesthetic vapour is collected from the vaporiser and delivered to the anaesthetic breathing circuit using oxygen, which is required to sustain the life of the patient. The oxygen flows through the anaesthetic vaporiser and carries the vapour to the anaesthetic breathing circuit.
Anaesthetic vaporisers must perform predictably and must be capable of delivering accurate, controlled dosages of anaesthetic vapour to the anaesthetic breathing circuit. For example, the anaesthetic dose of halothane which is administered to the lungs is between 1% and 2%. The lethal dose of halothane is around 3%-4%. It is therefore important that anaesthetic vaporisers are maintained and not subject to conditions which may damage them and render their output inaccurate.
Anaesthetic circuits vary in type. The aim is to deliver a controlled amount of anaesthetic and oxygen to a patient. Circuits include “rebreathing” systems which contain valves and soda lime (which removes the carbon-dioxide exhaled by the patient, so that the exhaled gas can be re-breathed) or non-rebreathing systems which have no soda-lime and where all exhaled gases are exhausted (eg T pieces, Bain circuits, Norman elbows).
Any anaesthetic delivery system will usually include a number of elements, as follows:
Oxygen supply, usually in the form of a pressurised vessel, with a regulator, for providing oxygen to the anaesthetic system; an oxygen flow meter for monitoring oxygen flows; a vaporiser connected in the oxygen flow line providing anaesthetic to the anaesthetic breathing circuit; an oxygen flush valve and lines to bypass the vaporiser to provide pure oxygen to the patient without anaesthetic; a pressure gauge to indicate the pressure in the patient's breathing circuit; various gas lines connecting together all the components in the circuit.
Some or all of these components may be mounted to a support structure, forming what is known as an “anaesthetic machines”. This support may provide mountings for the flow meter, the vaporiser and also mountings for the various gas lines connecting the anaesthetic delivery system. Anaesthetic machines tend to be of two types. A first type are large machines mounted on four wheel trolleys, usually with all components enclosed, and typically found in hospital surgeries. A second type is small machines which are portable and can be hand carried or fitted to a small mobile stand. One problem with present arrangements of portable anaesthetic machines is that because there are a number of relatively long gas flow lines, many of which remain exposed, they could become tangled and could even be pulled out during operation or transport of the system. This is dangerous. Further, any anaesthetic machine which has exposed gas lines is difficult to transport either to end user or between hospitals or after sale service of machine (eg to the end user after sale or service of the machine) without damage, as well as being inconvenient to use.